Would like to say Welcome to Runes of Magic. The dual class system works when you start the game you choose the class you want (some classes are bound to certain races). The class you choose in the beginning starts off as your primary class. Once you reach lvl 10 you can go to varanas and pick your secondary class. If you choose lets say rogue/priest you will get all the general skill of a rogue and the rogue specific skills of the rogue + the general skill of a priest. You can also switch and become a p/r if you choose to. At lvl 20 on both classes in this example rogue and priest you can choose another class. Ex: Rogue/Priest/Knight but you still only have the primary and secondary options so you can become R/K P/K K/R etc. You will not be forced to use the item shop if you do not want to. There are many hardcore f2p players on ROM that are completely end game and havent spent real money. It will be easier at times to spend real money but it is possible not to spend money, but your going to have to put in the time. The most populated server is Artemis but all servers are fun and you can create a toon on each server to try it out if you want. I love Reni and the ppl are awesome. For the class depends on what you want to do whether tank, dps or heal and whether you want to solo a lot of instances by yourself early on. Tanking would be knight/priest or knight/warrior and have seen warrior/knight and rogue/knight.
DPS: Melee: Rogue/Scout Warrior/Rogue Scout/Warden Warrior/Warden
Magic: Mage/Druid Mage/Priest (certain circumstances Priest/Scout)
Healing: Best healer Priest/Knight
For soloing I would say Mage/Priest is not bad. But to solo instances you will need to be certain lvl's ahead to be able to do it and gear will also be a factor.
Whatever class you choose if you dont like it create another and try it out. I hope you have fun at Runes of Magic
To better understand the skills for classes go here:
http://www.theromwiki.com/Runes_of_Magic_Wiki
For help on quests and list of gear:
http://www.runesdatabase.com/ 
I have zero clue how this works. Does it mean that I can take a super high damaging class like idk rouge and combine him with a priest to heal whenever I want? or take a mage and combine that with a warrior to get high defense and good DPS? I have played many MMOs before like WoW, maplestory LOTRO, Runescape (however, I only like MS
The other games gave me a headache on paying to play, though this game is free 
) Also will the Cash shop (or whatever the paying with real money) be something I am forced to do at the end? Like I will need to buy real money by the end? Also what sever should I join? Also can you help me chose a fun dual class. I'm think of Rouge and priest, however, im not sure. Thanks in advance for the help. 
Ill try R/P because it sounds fun. Thank you for the help and the links for extra info. 
